Over the course of both spacewalks, the pair will swap out a broken 780-pound (354-kilogram) ammonia pump with one of four spares stored on the space station's exterior. They'll also disconnect, then reconnect, the electrical and ammonia lines running between the pump and the station.

Astronauts on Earth are currently practicing the swap in a huge swimming pool at Johnson Space Center in Houston called the Neutral Buoyancy Lab.

Working in water simulates conditions the astronauts face in orbit, allowing NASA engineers to refine repair procedures and develop a time line, which will then be relayed to the spacewalkers aboard the ISS.

Coolant Failure a "Planned For" Anomaly

The space station's troubles began last Saturday, when an electrical spike tripped a circuit breaker in a pump for the station's coolant loop A. The coolant loop circulates superchilled liquid ammonia through huge radiators aboard the U.S. segment of the ISS to dissipate heat generated by electronics.

The ammonia pump failure is significant, because cooling is one of the station's major systems, aka the "Big 14," space station program manager Mike Suffredini said in a press conference earlier this week.

"This is an anomaly we knew someday would happen," Suffredini said. "It's an anomaly we have trained for, it's an anomaly we have planned for."

The station's six crew members—three U.S. astronauts and three Russian cosmonauts—are in no immediate danger, and a backup system, called coolant loop B, is functioning normally, said NASA spokesperson John Yembrick.

"All critical systems are functioning," Yembrick told National Geographic News. Some extra cables are now running through the inside of the station to connect electronics to coolant loop B, "but other than that, it's pretty much business as usual."

Worst Case: Move to Russian Segment of ISS

In the very unlikely event that coolant loop B fails before coolant loop A can be replaced, the station's crew will have to cloister in the Russian segment of the ISS, which has its own cooling system.

"That's a worst-case scenario, but this should all be behind us after [Wednesday's] spacewalk and the second pump is installed," Yembrick said.

The shutdown of coolant loop A forced the space station crew to quickly power down a variety of critical systems to prevent overheating and to move biological research samples between freezers.

"The crew transferred all the samples into the other freezer, so none of that science was lost," said NASA spokesperson Kelly Humphries.

In some ways, the cooling system failure couldn't have happened at a better time, mission managers noted.

NASA already had a spacewalk planned for Thursday to perform some external modifications to the station, so preparations were underway, spacewalk flight director Courtenay McMillan told reporters earlier this week.

"For Big 14 failures, in general, we typically allow for about two weeks to prepare," McMillan said.

"Since we already had [a spacewalk] coming up, we decided to take advantage of the fact that the crew already has the airlock and all the suit systems all ready to go—but this is a very aggressive time line for us."